A question on the abundance of the elements

How can scientists determine,calculate or understand that in the universe hydrogen is the most abundunt element as there are thousands of galaxy with their own planets. There might even an undiscovered element that could be most abundunt.

There are no obvious stable places left in the table of isotopes, any undiscovered elements on the periphery of the stable zone can be expected to have extremely short lifetimes, so they really cannot be abundant.

How can scientists determine,calculate or understand that in the universe hydrogen is the most abundunt element as there are thousands of galaxy with their own planets. There might even an undiscovered element that could be most abundunt.

Have a nice day.

One would have to sample galaxies/stars/planets to look at emission lines, and make some estimates of mass, in order to determine the relative abundance of elements in the cosmos.

Locally, we would have to take rock samples, and then make estimates on various geologic structures in order to estimate the relative abundance of elements.